Storing energy in the form of compressed gas has a long history and components tend to be well tested and reliable, and have long lifetimes. The general principle of compressed-gas or compressed-air energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If a body of gas is at the same temperature as its environment, and expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas will remain at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of high-pressure gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, expansion where no heat is exchanged between the gas and its environment—e.g., because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during expansion and compression, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
An efficient and novel design for storing energy in the form of compressed gas utilizing near isothermal gas compression and expansion has been shown and described in U.S. Pat. No. 7,832,207, filed Apr. 9, 2009 (the '207 patent) and U.S. Pat. No. 7,874,155, filed Feb. 25, 2010 (the '155 patent), the disclosures of which are hereby incorporated herein by reference in their entireties. The '207 and '155 patents disclose systems and techniques for expanding gas isothermally in staged cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. Mechanical energy from the expanding gas may be used to drive a hydraulic pump/motor subsystem that produces electricity. Systems and techniques for hydraulic-pneumatic pressure intensification that may be employed in systems and methods such as those disclosed in the '207 and '155 patents are shown and described in U.S. patent application Ser. No. 12/879,595, filed Sep. 10, 2010 (the '595 application), the disclosure of which is hereby incorporated herein by reference in its entirety.
In the systems disclosed in the '207 and '155 patents, reciprocal mechanical motion is produced during recovery of energy from storage by expansion of gas in the cylinders. This reciprocal motion may be converted to electricity by a variety of means, for example as disclosed in the '595 application as well as in U.S. patent application Ser. No. 12/938,853, filed Nov. 3, 2010 (the '853 application), the disclosure of which is hereby incorporated herein by reference in its entirety. The ability of such systems to either store energy (i.e., use energy to compress gas into a storage reservoir) or produce energy (i.e., expand gas from a storage reservoir to release energy) will be apparent to any person reasonably familiar with the principles of electrical and pneumatic machines.
During expansion of gas from storage in certain systems such as those disclosed in the '207 and '155 patents, the pressure of a quantity of gas within one chamber of a pneumatic or pneumatic-hydraulic cylinder exerts a force upon a piston and attached rod slidably disposed within the cylinder. The force exerted by the gas upon the piston and rod causes the piston and rod to move. The temperature of the gas undergoing expansion tends to decrease. To control the temperature of the quantity of gas being expanded within the cylinder (e.g., to hold it substantially constant, that is, to produce isothermal expansion), a heat-exchange liquid may be sprayed into the chamber containing the expanding gas. To prevent excess heat-exchange liquid from accumulating within the chamber, heat-exchange liquid may be removed continuously or episodically from the chamber. The liquid is conducted through a pipe to a pump that forces the liquid through a heat exchanger and back to the hydraulic cylinder, where the liquid is re-injected as a spray. The temperature of gas undergoing compression within the cylinder may be similarly controlled by circulation of the heat-exchange liquid.
In such an arrangement, portions of the cylinder assembly are in motion during either expansion or compression of gas within the cylinder. Consequently, if continuous circulation of the heat-exchange liquid is to be maintained, the pipe conveying the heat-exchange liquid generally flexes while the piston moves. A flexible pipe that flexes during operation is herein termed a hose. Flexure of a hose subjects its constituent materials, its connection points, and possibly other components (e.g., rod and/or rod gaskets) to time-varying forces. Such forces tend to shorten the lifespans of components subjected to them. Hence, there is a need for systems enabling the circulation of heat-exchange fluids into, within, and/or out of compressed-gas energy storage and recovery systems without utilization of flexible hose.